Recently, liquid crystal displays have been widely used for, for example, office automation devices such as word processors and personal computers, mobile information devices such as PDAs, and camera-equipped VTRs including liquid crystal monitors owing to their features of being thin and consuming low power.
Unlike light emitting displays such as CRTs (cathode ray tubes), PDPs (plasma display panels) and EL (electroluminescence) devices, non-light emitting displays represented by liquid crystal displays do not emit light themselves but control a transmitted amount or a reflected amount of externally emitted light to display characters and images.
The above-mentioned liquid crystal displays are roughly classified into transmission type liquid crystal displays and reflection type liquid crystal displays.
Transmission type liquid crystal displays perform display using light of an illumination device (a so-called backlight) provided behind a liquid crystal panel, whereas reflection type liquid crystal displays perform display using ambient light. Some known reflection type liquid crystal displays include an illumination device for improving the display quality for the case where a sufficient strength of light is not obtained. Such an illumination device is called a “front light”, as opposed to a “backlight” which is an illumination device of a transmission type liquid crystal display.
Most of the transmission type liquid crystal displays practically used today include a pair of polarizers facing each other with a liquid crystal cell interposed therebetween. Most of the reflection type liquid crystal displays practically used today include a polarizer provided on the viewer side of the liquid crystal cell. Therefore, in the case where illumination light emitted from an illumination device (a backlight or a front light) is in a randomly polarized state, about 50% of the illumination light is absorbed by the polarizer before being incident on the liquid crystal cell.
In order to reduce the amount of light absorbed by the polarizer and thus to improve the light utilization efficiency, illumination devices for selectively emitting light of a predetermined polarization direction have been proposed.
For example, Japanese Laid-Open Patent Publication No. 9-5739 and Tanase and five others, “A New Backlighting System with a Polarizer Light Pipe for Enhanced Light Output from LCDs”, SID 97 DIGEST, pp. 365-368, disclose an illumination device for emitting light of a specified polarization direction, utilizing that the reflectance at an interface between transparent mediums having different refractive indices from each other has polarization dependency. FIG. 43 and FIG. 44 schematically show an illumination device 740 disclosed in Japanese Laid-Open Patent Publication No. 9-5739 and a liquid crystal display 700 including the illumination device 740 as a backlight, respectively.
The liquid crystal display 700 includes a transmission type liquid crystal display panel 710 and the illumination device (backlight) 740 provided on the rear side of the liquid crystal display panel 710.
The liquid crystal display panel 710 includes a pair of substrates 711 and 712, a liquid crystal layer 713 provided between the pair of substrates 711 and 712, and a pair of polarizers 715a and 715b provided outside the pair of substrates 711 and 712. The liquid crystal display panel 710 performs display by modulating light, emitted from the illumination device 740 and incident on the liquid crystal display panel 710 via the polarizer 715b, by the liquid crystal layer 713 and thus controlling the amount of light transmitted through the polarizer 715a. 
The illumination device 740 includes a light source 741, a lightguide element 720, and a reflection film 742 provided so as to surround the light source.
The lightguide element 720 includes a first side surface (incidence surface) 720a on the side of the light source 741, a second side surface 720b facing the first side surface 720a, an outgoing surface 720c from which light incident from the light source 741 goes out, and a counter surface 720d facing the outgoing surface 720c. A λ/4 plate (quarter-wave plate) 732 and a reflection plate 734 are provided in the vicinity of the second side surface of the lightguide element 720, and a reflection plate 736 is provided in the vicinity of the counter surface 720d of the lightguide element 720.
The lightguide element 720 is formed of a lightguide plate 721 and a lightguide sheet 723 attached to each other. The lightguide sheet 723 is formed of transparent amorphous layers 723a and 723b which have different refractive indices from each other and are alternately stacked at a predetermined angle.
Light, which is emitted from the light source 741 and is incident on the inside of the lightguide element 720 via the incidence surface 720a, is propagated toward the second side surface 720b while being totally reflected between the outgoing surface 720c and the counter surface 720d repeatedly. A part of the light propagated in the lightguide element 720 is reflected by interfaces between the amorphous layers 723a and 723b forming the lightguide sheet 723, and goes out from the outgoing surface 720c toward the liquid crystal display panel 710.
It is known that the reflectance is different in accordance with the polarization direction at the interfaces between the amorphous layers having different refractive indices from each other. Especially when light is incident on such an interface at a specific angle of incidence which is referred to as the “Brewster angle”, the reflectance of P polarized light is zero and only S polarized light is reflected.
Accordingly, by stacking the amorphous layers 723a and 723b forming the lightguide sheet 723 so as to be at an angle closer to the Brewster angle with respect to the outgoing surface 720c of the lightguide element 720, the reflectance of first polarized light vibrating in a direction perpendicular to the direction in which the amorphous layers 723a and 723b are alternately repeated (vibrating in the direction vertical to the sheet of FIG. 44) can be made high and the reflectance of second polarized light vibrating in a direction parallel to the direction in which the amorphous layers 723a and 723b are alternately repeated (vibrating in the direction parallel to the sheet of FIG. 44) can be made low. Thus, the illumination light going out from the lightguide element 720 can have polarization characteristics.
The λ/4 plate 732 and the reflection plate 734 located in the vicinity of the second side surface 720b of the lightguide element 720 are provided in order to realize the following: the polarization direction of light, which does not go out from the outgoing surface 720c of the lightguide element 720 and reaches the second side surface 720b, is rotated and such light is again incident on the inside of the lightguide element 720, and thus the light utilization efficiency is improved. The reflection plate 736 located in the vicinity of the counter surface 720d of the lightguide element 720 is provided in order to reflect the illumination light, reflected toward the lightguide element 720 by the liquid crystal display panel 710, back toward the liquid crystal display panel 710.
In the liquid crystal display 700, light of a specific polarization direction is selectively emitted from the illumination device 740 as described above. Therefore, absorption of light by the polarizer 715b of the liquid crystal display panel 710 can be suppressed, and thus the light utilization efficiency is improved.
Japanese PCT National Phase Laid-Open Publication No. 10-508151, Japanese PCT National Phase Laid-Open Publication No. 2001-507483, S. M. P. Blom and two others, “Towards Polarized Light Emitting Back Lights: Micro-structured Anisotropic Layers”, Asia Display/IDW '01, pp. 525-528, and Henri J. B. Jagt and three others, “Micro-structured Polymeric Linearly Polarized Light Emitting Lightguide for LCD Illumination”, SID 02 DIGEST, pp. 1236-1239; disclose an illumination device for emitting light of a specific polarization direction, utilizing that the reflectance at an interface between a material having an isotropic refractive index and a material having an anisotropic refractive index has polarization dependency. FIGS. 45(a), (b) and 46 schematically show an illumination device 800 disclosed in the Asia Display/IDW '01, pp. 525-528.
The illumination device 800 includes a light source 810, a lightguide element 820, and a reflection film 812 provided so as to surround the light source 810.
The lightguide element 820 includes a first side surface (incidence surface) 820a on the side of the light source 810, a second side surface 820b facing the first side surface 820a, an outgoing surface 820c from which light incident from the light source 810 goes out, and a counter surface 820d facing the outgoing surface 820c. 
The lightguide element 820 is formed of an isotropic layer 821 formed of a material having an isotropic refractive index and an anisotropic layer 823 formed of a material having an anisotropic refractive index which are stacked on each other. A surface of the isotropic layer 821 on the side of the anisotropic layer 823 has grooves having a V-shaped cross section formed therein at a constant pitch, and a surface of the anisotropic layer 823 on the side of the isotropic layer 821 has projections engageable with the V-shaped grooves formed thereon. Thus, the cross section of the interface between the isotropic layer 821 and the anisotropic layer 823 is wave-like. The anisotropic layer 823 is designed such that only a refractive index ne in a specific direction is different from a refractive index n of the isotropic layer 821 and a refractive index no in the other directions is almost the same as the refractive index n of the isotropic layer 821.
Light, which is emitted from the light source 810 and is incident on the inside the lightguide element 820 via the incidence surface 820a, is propagated toward the second side surface 820b while being totally reflected between the outgoing surface 820c and the counter surface 820d repeatedly. A part of the light propagated in the lightguide element 820 is reflected by parts of the interface between the anisotropic layer 823 and the isotropic layer 821, the parts being inclining with respect to the outgoing surface 820c, and goes out from the outgoing surface 820c. 
At the interface between the anisotropic layer 823 and the isotropic layer 821, only first polarized light vibrating in a direction in which the refractive indices thereof are different from each other is reflected, and second polarized light vibrating in a direction in which the refractive indices thereof are almost the same is not reflected. Therefore, the illumination light going out from the lightguide element 820 can have polarization characteristics.
In the liquid crystal display 800, light of a specific polarization direction is selectively emitted from the outgoing surface 820c as described above. Therefore, the light utilization efficiency can be improved.
Japanese PCT National Phase Laid-Open Publication No. 10-508151 also discloses an illumination device for emitting light of a specific polarization direction, utilizing that the reflectance at an interface between an isotropic layer and an anisotropic layer has polarization dependency, like the illumination device 800 shown in FIGS. 45(a), (b) and 46. Japanese PCT National Phase Laid-Open Publication No. 10-508151 further discloses that as shown in FIGS. 45(a) and 46, the light utilization efficiency can be further improved by providing a depolarizing reflection plate 832 in the vicinity of the second side surface 820b of the lightguide element 820. The depolarizing reflection plate 832 depolarizes the second polarized light which is not reflected at the interface between the anisotropic layer 823 and the isotropic layer 821 and causes a part of such light to be incident again on the lightguide element 820 as first polarized light. Therefore, the second polarized light can be utilized as the illumination light.
Japanese Laid-Open Patent Publication No. 9-218407 discloses an illumination device for emitting light of a specific polarization direction, utilizing the polarization dependency of diffraction in an arranged grating formed at an interface between an isotropic layer (a layer formed of a material having an isotropic refractive index) and an anisotropic layer (a layer formed of a material having an anisotropic refractive index). FIGS. 47(a), (b) and 48 schematically show an illumination device 900 disclosed in Japanese Laid-Open Patent Publication No. 9-218407.
The illumination device 900 includes a light source 910, a lightguide element 920, and a reflection film 912 provided so as to surround the light source 910.
The lightguide element 920 includes a first side surface (incidence surface) 920a on the side of the light source 910, a second side surface 920b facing the first side surface 920a, an outgoing surface 920c from which light incident from the light source 910 goes out, and a counter surface 920d facing the outgoing surface 920c. 
The lightguide element 920 is formed of an isotropic layer 921 formed of a material having an isotropic refractive index and an anisotropic layer 923 formed of a material having an anisotropic refractive index which are stacked on each other. The anisotropic layer 923 is designed such that only a refractive index ne in a specific direction is different from a refractive index n of the isotropic layer 921 and a refractive index no in the other directions is almost the same as the refractive index n of the isotropic layer 921. An interface between the isotropic layer 921 and the anisotropic layer 923 is rectangular wave-like, and the interface between the isotropic layer 921 and the anisotropic layer 923 acts as an arranged grating. A phase plate 932 and a reflection plate 934 are provided on the side of the counter surface 920d of the lightguide element 920.
Light, which is emitted from the light source 910 and is incident on the inside the lightguide element 920 via the incidence surface 920a, is propagated toward the second side surface 920b while being totally reflected between the outgoing surface 920c and the counter surface 920d repeatedly. A part of the light propagated in the lightguide element 920 is diffracted by the arranged grating formed at the interface between the anisotropic layer 923 and the isotropic layer 921, and goes out from the outgoing surface 920c. 
At the interface between the anisotropic layer 923 and the isotropic layer 921, only first polarized light vibrating in a direction in which the refractive indices thereof are different from each other is reflected, and second polarized light vibrating in a direction in which the refractive indices thereof are almost the same is not reflected. Therefore, the illumination light going out from the lightguide element 920 can have polarization characteristics.
In the liquid crystal display 900, light of a specific polarization direction is selectively emitted from the outgoing surface 920c as described above. Therefore, the light utilization efficiency can be improved.
Japanese Laid-Open Patent Publication No. 9-218407 also describes that the second polarized light which is not diffracted by the arranged grating is converted into the first polarized light by the anisotropic layer 923 and the phase plate 932 while being propagated in the lightguide element 920 toward the second side surface 920b, and therefore the second polarized light also can be utilized as the illumination light.
However, the illumination devices described above have the following problems.
In the illumination device 740 shown in FIG. 43 and FIG. 44 and the illumination device 800 shown in FIGS. 45 and 46, the second polarized light which is not directly reflected by the interface between the amorphous layers 723a and 723b, or the interface between the isotropic layer 821 and the anisotropic layer 823, is converted into the first polarized light by the λ/4 plate 732 and the reflection plate 734 provided in the vicinity of the second side surface 720b of the lightguide element 720, or the depolarizing reflection plate 832 provided in the vicinity of the second side surface 820b of the lightguide element 820.
A transparent resin such as polymethylmethacrylate or polycarbonate generally used as a material of a lightguide element has slight birefringence. In order to convert the second polarized light which has reached the second side surface 720a of the lightguide element 720, or the second side surface 820a of the lightguide element 820, into the first polarized light by the λ/4 plate 732 and the reflection plate 734, or the depolarizing reflection plate 832, the birefringence of the lightguide element 720 or 820 needs to be suppressed sufficiently low for the following reason. When the lightguide element 720 or 820 has a large birefringence, the second polarized light propagated in the lightguide element 720 or 820 is partially depolarized and reaches the second side surface as the first polarized light. Such light is converted into second polarized light by the λ/4 plate 732 and the reflection plate 734 or the depolarizing reflection plate 832. As a result, such light is not propagated toward the outgoing surface 720c or 820c after being incident again on the lightguide element 720 or 820.
Therefore, in the illumination devices 740 and 800, the lightguide elements 720 and 820 each need to be formed of a material having a sufficiently small birefringence, which restricts the range of usable materials.
Recently, liquid crystal displays have been remarkably reduced in thickness, to the extent that the lightguide element 720 or 820 may be about 0.7 mm to 0.8 mm thick at the second side surface 720b or 820b. For the production-related reasons, it is very difficult to locate the λ/4 plate 732 and the reflection plate 734, or the depolarizing reflection plate 832, in the vicinity of such a second side surface 720b or 820b of the lightguide element 720 or 820 with high precision. Considering that the liquid crystal displays will become thinner in the future, such a structure is not practical.
Regarding the illumination device 900 shown in FIG. 47 and FIG. 48, patent document 4 describes that the second polarized light is converted into the first polarized light by the anisotropic layer 923. However, it is theoretically impossible that the second polarized light is converted into the first polarized light by the birefringence of the anisotropic layer 923, because the first polarized light and the second polarized light respectively correspond to ordinary light and extra ordinary light for the anisotropic layer 923 in the illumination device 900. Therefore, in the illumination device 900, the second polarized light must be converted into the first polarized light only by the phase plate 932.
However, Japanese Laid-Open Patent Publication No. 9-218407 describes no practical specifications of the phase plate 932, for example, the anisotropy of the refractive index, thickness, and the direction of the optical axis (slow axis or fast axis). Japanese Laid-Open Patent Publication No. 9-218407 does not disclose any knowledge required for efficiently converting the second polarized light into the first polarized light.
In addition, in the illumination device 900, light is diffracted toward the counter surface 920d by the arranged grating formed at the interface between the isotropic layer 921 and the anisotropic layer 923 as well as toward the outgoing surface 920c. Therefore an unnegligible amount of light goes out from the counter surface 920d, which decreases the light utilization efficiency. When the illumination device 900 is used as a front light, light goes out toward the viewer and thus the display quality is deteriorated.
As described above, an illumination device capable of causing light from a light source to go out as light of a specific polarization direction sufficiently efficiently has not been developed.
The present invention, made in light of the above-described problems, has an object of providing an illumination device capable of causing light from a light source to go out as light of a specific polarization direction sufficiently efficiently, an image display apparatus including the same, and a lightguide element.